return a boolean after promise has been resolved/rejected?

Question

I have a function that works absolutely fine. I just want to return true or false depending on the promise.

 //I want this function to return a simple true or false!!!
function isAppOnline() {
            var is_connected = connectivityMonitor.isInternetConnected();
            is_connected.then(function(result) {
                console.log('INTERNET_CHECK_API : app online');//works fine
                return true;//this is not being returned
            }, function(error) {
                console.log('INTERNET_CHECK_API : app offline');//works fine
                return false;//this is not being returned
            });
        }

But when I call this function,

is_online = isAppOnline();

is_online is always undefined . Why is the function not able to return a simple boolean value?

Update :

This is what I am trying to do : I simply want to open a popup which notifies the user that he is offline. I am calling the function isAppOnline periodically after 10secs. This function is already using a promise in my factories . I dont want to overcomplicate things but its important to me that this function returns a boolean so based on this,I can take my actions accordingly.

Answer 1

EDIT: For ES2017 If your one of the lucky souls who gets to use ES2017 then you can use the new await/async keywords. They are pretty brilliant and allow you to write async code that reads synchronous. (It is still promises under the hood, just unboxing of them).

function isOnline() {
    return Promise.resolve(true);
}

async function Main() {
    const online = await isOnline();

  console.log(online);
}

Main();

fidle

---

Because it is asynchronous. Your isAppOnline method returns before your promise has resolved.

I presume is making some form of AJAX call to check network connectivity so there it will have to wait for it to respond. JavaScript is single threaded, if that thread locked up waiting for that request to respond so it could be synchronous your whole JavaScript would pause until it returns. Not good.

So if you want the caller of isAppOnline to know the result you have to options. Either passing it a call back or return the promise (better option)

function isAppOnline(cb) {
    var is_connected = connectivityMonitor.isInternetConnected();
    is_connected.then(function(result) {
        console.log('INTERNET_CHECK_API : app online');//works fine
        cb(true);
    }, function(error) {
        console.log('INTERNET_CHECK_API : app offline');//works fine
        cb(false);
    });
}

//better option
function isAppOnline() {
    return connectivityMonitor.isInternetConnected().then(function(result) {
        console.log('INTERNET_CHECK_API : app online');//works fine
        return true;
    }, function(error) {
        console.log('INTERNET_CHECK_API : app offline');//works fine
        return false;
    });
}

//used as
isAppOnline().then(function (isOnline) {
    console.log('Is it online?', isOnline);
});